In conventional ships, a two-stroke low speed diesel engine is used. The diesel engine can output at low speeds and is driven, directly connected to a propeller.
In recent years, natural gas with low NOx and SOx emissions has received attention as a fuel for low speed diesel engines. By injecting high pressure natural gas as a fuel into a combustion chamber of a low speed diesel engine and burning it, an output can be obtained with high heat efficiency.
For example, a reciprocating pump is driven by converting the rotational motion to the reciprocating motion using a crankshaft. In a case where a piston of the reciprocating pump is driven using the crankshaft, since the piston stroke is determined by the crankshaft, it is not possible to freely adjust the piston stroke. In addition, in a case where a plurality of reciprocating pumps is driven by an identical crankshaft, it is difficult to independently control the individual reciprocating pumps.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2005-504927 (“JP 2005-504927”) describes a device that boosts liquid fuel using a reciprocating pump and supplies the boosted fuel to the engine. In the device in JP 2005-504927, a piston of the reciprocating pump is driven in the left-right direction, and a “linear hydraulic motor” (hydraulic cylinder unit) is used as a linear actuator that drives a piston. In JP 2005-504927, the moving direction of the piston of the reciprocating pump is switched by switching, with a direction switching valve, the direction of the hydraulic oil supplied from the hydraulic pump to the hydraulic cylinder unit. With the use of the hydraulic cylinder unit, it is possible to drive the reciprocating pump at a lower speed than the case where the crankshaft is used. Moreover, this method has an advantage that the piston stroke can be controlled so as to allow the piston to move at a constant speed.
Meanwhile, in a case of adjusting the fuel supply amount in accordance with the load of an internal combustion engine in a fuel supply device configured to supply fuel to the internal combustion engine using a reciprocating pump driven by a crankshaft, there is a need to change a reciprocating cycle of the reciprocating pump. Unfortunately, however, changing the reciprocating cycle of the reciprocating pump might induce fluctuation of the pressure and the flow rate of the fuel according to the reciprocating cycle, leading to a difficulty in achieving an optimum flow rate, particularly when the output is low.
In the case of the crankshaft driven reciprocating pump, in particular, lubrication conditions of a joining rod for a cylinder side bearing can be very critical in slow rotation because the load from the joining rod is kept in one direction, leading to a high possibility of generating abnormal abrasion of the bearing and seizure. To avoid this, in a case where a low output is needed in the crankshaft driven reciprocating pump, bypassing a surplus amount to the necessary amount while the pump ejection amount keeps too much is required. This often causes a power loss.